Aside from the obvious bit size changes (e.g. i8 -> i16 -> i32 -> i64, or f32 -> f64), there is no "hierarchy" of types. Not all ints are representable as floats. u64 can represent up to 2^64 - 1, but f64 can only represent up to 2^53 with integer-level precision. This issue may be subtle, but Rust is all about preventing subtle footguns, so it does not let you automatically "promote" integers to float - you must be explicit (though usually all you need is an `as f64` to convert).
> Aside from the obvious bit size changes (e.g. i8 -> i16 -> i32 -> i64, or f32 -> f64), there is no "hierarchy" of types.
Depends on what you want from such a hierarchy, of course, but there is for example an injection i32 -> f64 (and if you consider the i32 operations to be undefined on overflow, then it’s also a homomorphism wrt addition and multiplication). For a more general view, various Schemes’ takes on the “numeric tower” are informative.
Virgil allows the maximum amount of implicit int->float injections that don't change values and allows casts (in both directions) that check if rounding altered a value. It thus guarantees that promotions and (successful) casts can't alter program behavior. Given any number in representation R, promotion or casting to type N and then casting back to R will return the same value. Even for NaNs with payloads (which can happen with float <-> double).
